SECT. 1] THE MAGNETIC FIELD OVEB, THE OCEANS 203 



the change in crustal structure across the Mendocino Fault might then be that 

 transcurrent faulting has brought dissimilar parts of the crust opposite one 

 another. Study of the way in which the more than 1000 km of unmatched 

 magnetic pattern on the south side of the Pioneer Fault between about 135° 

 and 145°W is absorbed in the westward direction, and the discovery of similar 

 patterns in other parts of the oceans, would do much towards narrowing down 

 the range of tenable hypotheses. 



7. The Continental Shelves 



The continental shelves are structurally more closely related to the continents 

 than to the deep ocean. Magnetic surveys over them may be expected to show 

 the same features as do those on land and to lead to the same ambiguities in 

 interpretation. Plots of the field frequently show well-marked linear trends, 

 such as those shown in Fig. 17, but it is impossible to say from magnetic 

 measurements alone whether the regions of high magnetic intensity are regions 

 where the basement is shallower than it is elsewhere, or whether the magnetic 

 trends represent the trends of intrusions of highly magnetic material within 

 the basement. Sometimes the "roughness" of the magnetic plot may give a 

 clue that the basement is shallow, but numerical estimates of depth are not to 

 be relied on. In fact, seismic work suggests that the area of large anomalies 

 shown in Fig. 17 to the west of Brittany is an area where the basement is 

 shallower than it is farther to the north, where the magnetic plot is relatively 

 featureless (Day et al., 1956). 



Even if the cause of the anomalies is not known in detail, a magnetic map 

 may show trends that run j)arallel to other features of the area; Fig. 17, for 

 example, shows persistent ENE-WSW trends parallel to the centre line of the 

 English Channel, the folding of south-west Ireland and the line of bathyliths 

 from Dartmoor to the Scillies. The west coast of Europe is intersected or 

 approached by folding of three periods, the Caledonian, the Hercynian and the 

 Alpine ; when the area of survey has been extended, it is to be hoped that their 

 course and direction across the continental shelf can be plotted. It would be of 

 particular interest to know whether the Caledonian trend can be traced to the 

 north and west of Ireland and whether the Alpine folding in the south of Spain 

 sends a branch to meet the rather vague ridge and line of earthquake epicentres 

 that runs from the Azores towards Gibraltar. 



Perhaps the most important use for the magnetometer on and near the 

 continental shelf is to determine whether the structural lines of the continents 

 run out into the deep ocean. This can best be investigated in an area such as 

 western Europe where powerful structures strike out more or less perpendicular 

 to the continental margin. Many other areas, such as that to the north of 

 Newfoundland, should also be investigated. Whether the structural lines from 

 the continents persist into the oceans or are truncated at the continental margin 

 is one of the fundamental questions of geology ; unfortunately, the magnetic 

 evidence is at present insufficient to decide it. 



